1. Field of the Invention
The invention relates to a wheel truck of a railroad freight car.
2. Description of the Related Art
As a critical part of a railroad freight car, a typical wheel truck includes two side frame assemblies and a bolster assembly. Journal-box guides disposed on two ends of the frame assembly are fixed on a front wheel pair and a rear wheel pair via roller bearing adapters, respectively. The bolster assembly has two ends, each of which is mounted in a central square box of the side frame assembly via a spring suspension device for supporting the load from the bolster assembly. The bolster assembly includes a lower center plate in the center and two lower side bearings on two ends. The lower center plate and the lower side bearings are matched with an upper center plate and two upper side bearings on the lower base of the freight car for supporting the weight of the freight car.
In early supporting structure of the freight car, the lower center plate of the bolster assembly supported all loads of the car body, whereas the lower side bearings assisted for positioning. Thereafter, in order to improve the critical speed of an empty freight car, the lower center plate was improved as a primary bearing structure, and the lower side bearing was improved to assist for supporting. The friction between the upper and lower side bearings can act as a resistance during the turnaround of the wheel truck to meet the requirement of speed-raising.
In above descriptions, the supporting achieved totally by the lower center plate and the supporting achieved by the combination of the lower center plate and the lower side bearings are commonly called center plate-type supporting. The wheel truck having the center plate-type supporting is advantageous in that when crossing curved tracks, the wheel truck is flexible in turning around, and the load is uniformly distributed on the wheels. However, it has defects that the vertical load of the body is directly applied on the center of the bolster assembly, and transmitted to the square boxes via the bolster assembly, which results in a large bending moment and sectional area of the bolster assembly. Correspondingly, the weight and the production cost of the assembly are increased, and the center plate has a low stability in rolling. Thus, it is very significant to eliminate the large bending moment produced by the bolster assembly and improve the performance and the running stability of the freight car when crossing curved tracks.